Optical data recording medium and material for heat-resistant protection layer for the same

ABSTRACT

In a material for a heat-resistant protection layer and constituting one of the components of a phase variation type recording medium, at least one compound having a thermal conductivity of higher than 10 W/m.deg inclusive in a bulk state is contained. This kind of material realizes a phase variation type optical data recording medium having a high erasure ratio and allowing data to be repeatedly recorded and erased a number of times by small power even at the time of high-speed recording and erasure.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to an optical data recording mediumfor an apparatus operable with optical memories. More particularly, thepresent invention is concerned with a so-called phase variation typeoptical data recording medium in which a material constituting arecording layer varies in phase when illuminated by a light beam tothereby allow data to be recorded, reproduced, and rewritten, and amaterial for a heat-resistant protection layer for the production of themedium.

[0002] A recording medium of the type described belongs to a family ofconventional optical memory media which selectively allow data to berecorded, reproduced or erased when illuminated by a light beam,particularly a laser beam. The phase variation type recording medium isbased on the transition between the crystal phase and the non-crystalphase or between the crystal phases. This type of recording medium has,among others, an overwriting capability using a single beam; it isdifficult to provide a magnetooptical memory with this kind ofcapability. This, coupled with the fact that the recording mediumsimplifies optics to be built in a drive, is accelerating the study anddevelopment of such a recording medium.

[0003] U.S. Pat. No. 3,530,441, for example, teaches a phase variationtype recording medium using a so-called chalcogen alloy, e.g., Ge-Te,Ge-Te-S, Ge-Se-Sn, Ge-Te-S, Ge-Se-Sb, Ge-As-Se, In-Te or Se-As. Toenhance stability and rapid crystallization, Au, Sn and Au, or Pd may beadded to Ge-Te. The Au, Sn and Au, and Pd addition schemes arerespectively disclosed in Japanese Patent Laid-Open Publication Nos.61-219692, 61-270190, and 62-19490. Further, to improve the repeatedrecording and erasing ability, Ge-Te-Se-Sb and Ge-Te-Sb may each beprovided with a particular composition ratio, as proposed in JapanesePatent Laid-Open Publication Nos. 62-73438 and 63-228433. However, noneof these prior art schemes satisfies all the characteristics required ofthe phase variation type rewritable optical memory medium. Particularly,there is a keen demand for measures to improve the recording and erasingsensitivity, to obviate the decrease in erasure ratio ascribable toincomplete erasure at the time of overwriting, and to extend the life ofrecorded and non-recorded portions.

[0004] Japanese Patent Laid-Open Publication No. 63-251290 proposes arecording medium having a recording layer in the form of a single layerof substantially ternary or higher compound. In this document, thesingle layer of substantially ternary or higher compound refers to alayer containing a compound having a ternary or higher stoichiometriccomposition (e.g. In₃SbTe₂) by more than 90 atomic percent inclusive.The above document recites that the recording layer with such acomposition improves the recording and erasing characteristic. However,this kind of implementation has a problem that the erasure ratio is low,and a problem that the laser power necessary for recording and erasingdata has not been sufficiently lowered yet. Under these circumstances,an optical data recording medium having a high erasure ratio andsensitivity and a desirable repetition characteristic is called for.

[0005] In light of the above, some different materials have beendeveloped for a protection layer feasible for a recording medium. Forexample, there may be used ZnS·SiO₂ (Japanese Patent Laid-OpenPublication No. 4-74785), SiN or A1N (Japanese Patent Laid-OpenPublication No. 63-259855 and Japanese Patent Publication No. 4-74785).However, even with any combination of these materials, it is impossibleto satisfy all the characteristics required of the optical recordingmedium.

SUMMARY OF THE INVENTION

[0006] It is therefore an object of the present invention to provide aphase variation type data recording medium having a high erasure ratioand allowing data to be repeatedly recorded and erased a number of timesby low power even at the time of high-speed recording and erasure, and amaterial for a heat-resistant protection layer for the production of themedium.

[0007] In accordance with the present invention, in a material for aheat-resistant protection layer and constituting one of a plurality ofcomponents of a phase variation type recording medium, at least onecompound having a thermal conductivity of higher than 10 W/m.deginclusive in a bulk state is contained.

[0008] Also, in accordance with the present invention, in an opticaldata recording medium having a substrate and a heat-resistant protectionlayer, a recording layer and a reflective heat radiation layersequentially stacked on the substrate, the recording layer mainlyconsists of Ag, In, Sb and Te, and the heat-resistant protection layercontains at least one compound having a thermal conductivity of higherthan 10 W/m.deg inclusive in a bulk state.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The above and other objects, features and advantages of thepresent invention will become more apparent from the following detaileddescription taken with the accompanying drawings in which:

[0010]FIG. 1 shows a relation between the light-to-heat conversionefficiency a of a recording layer and the thermal conductivity κ of aprotection layer; and

[0011]FIG. 2 shows a specific structure of an optical data recordingmedium in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] I conducted a series of extended researches and experiments andfound materials for a heat-resistant protection layer which are usefulfor achieving the previously stated object. The present invention isbase on such materials. The materials in accordance with the presentinvention are characterized by the following:

[0013] (1) The materials are each used as one component of a phasevariation type recording medium using the phase variation of a recordingmaterial. Each material contains at least one compound whose thermalconductivity in the bulk state is higher than 10 W/m.deg inclusive;

[0014] (2) In the above characteristic (1), the one least one compoundis selected from a group consisting of aluminum oxide, titanium oxide,magnesium oxide, yttrium oxide, gallium nitride, silicon nitride,aluminum nitride, and silicon carbide; and

[0015] (3) In the characteristic (1), the component is implemented bythe combination of zinc oxide, aluminum oxide, titanium oxide, magnesiumoxide, yttrium oxide, gallium nitride, silicon nitride, aluminum nitrideand/or silicon carbide, and silicon oxide.

[0016] Also, the optical data recording medium in accordance with thepresent invention is characterized by the following:

[0017] (4) The medium has a substrate on which a heat-resistantprotection layer, a recording layer and a reflective heat radiationlayer are sequentially stacked. The recording layer is mainlyconstituted by Ag, In, Sb and Te while the protection layer isimplemented by the component or components mentioned in any one of theabove items (1) through (3).

[0018] The present invention will be described more specificallyhereinafter.

[0019] With the heat-resistant protection layer whose thermalconductivity in the bulk state is higher than 10 W/m.deg inclusive, therecording medium of the present invention achieves high sensitivity andallows data to be stably recorded and erased a number of timesrepeatedly. This unprecedented advantage presumably stems from thefollowing. Generally, to record data in a phase variation type opticalrecording medium, amorphous portions are formed in a recording layer ofthe medium. Amorphous portions cannot be formed unless the recordinglayer is heated to above its melting point and then cooled at asufficiently high speed. On the other hand, it is necessary that therecording layer, except for its recorded portions, be protected from theinfluence of heat as far as possible; otherwise, the boundary betweenthe recorded portions and non-recorded portions would be unclear, or therecorded portions would even be crystallized and erased. The temperatureelevation condition for the recorded portions and that for the otherportions which should be maintained at normal temperature can beeffectively distinguished if excessive heat generated in the recordinglayer is released toward the protection layer having a high thermalconductivity, i.e., heat transfer within the layer or film is reduced.This insures temperature elevation to above the melting point and thesufficient cooling speed.

[0020]FIG. 1 shows a relation between the light-to-heat conversionefficiency α of the recording layer and the thermal conductivity κ ofthe protection layer. In FIG. 1, sensitivities 1 through 5 sequentiallyincrease in the ascending order. As shown, the temperature elevation andcooling conditions of the recording layer are mainly determined by thebalance between the conversion efficiency a of the recording layer andthe thermal conductivity κ of the protection layer. When the thermalconductivity κ is excessively high, compared to the conversionefficiency α, the temperature rises above the melting point of therecording layer. This results in the need for great laser power which,in turn, lowers the sensitivity. At the same time, crystallization isobstructed with the result that the erasure ratio is lowered and impedesstable and repeated recording and erasure. On the other hand, if thethermal conductivity κ is excessively high, compared to the conversionefficiency α, excessive heat is accumulated in the recording layer andmakes it difficult to set up the cooling speed high enough for theformation of amorphous. Consequently, more than necessary power must beapplied in order to increase the apparent cooling speed, again resultingin the need for great laser power. In this case, the interfaces betweenthe consecutive layers of the medium are more damaged by the heat, sothat the stable and repeated recording and erasure is difficult toachieve. It follows that for high sensitivity, high carrier-to-noise(C/N) ratio and erasure ratio, and stable repeated recording, it isimportant that the protection layer be provided with a thermalconductivity matching the light-to-heat conversion efficiency of therecording layer. The protection layer in accordance with the presentinvention meets the above requirement.

[0021] Materials suitable for the heat-resistant protection layer havestated earlier in the items (2) and (3). Regarding the item (3),preferable contents in molar ratio are as follows:

[0022] (i) As to a material mainly consisting of silicon oxide and zincoxide, 3% to 50% of zinc oxide;

[0023] (ii) As to a material mainly consisting of silicon oxide andaluminum oxide, 5% to 95% of aluminum oxide;

[0024] (iii) As to a material mainly consisting of silicon oxide andtitanium oxide, 10% to 98% of titanium oxide;

[0025] (iv) As to a material mainly consisting of silicon oxide andmagnesium oxide, 3% to 45% of magnesium oxide;

[0026] (v) As to a material mainly consisting of silicon oxide andyttrium oxide, 10% to 80% of yttrium oxide;

[0027] (vi) As to a material mainly consisting of silicon oxide andgallium nitride, 1% to 30% of gallium nitride;

[0028] (vii) As to a material mainly consisting of silicon oxide andsilicon nitride, 10% to 85% of silicon nitride;

[0029] (viii) As to a material mainly consisting of silicon oxide andaluminum nitride, 1% to 50% of aluminum nitride;

[0030] (ix) As to a material mainly consisting of silicon oxide andsilicon carbide, 5% to 50% of silicon carbide; and

[0031] (x) As to a material mainly consisting of silicon oxide andtitanium carbide, 10% to 85% of titanium carbide.

[0032] Referring to FIG. 2, a specific configuration of the optical datarecording medium in accordance with the present invention is shown. Asshown, the medium has a substrate 1 on which a lower heat-resistantprotection layer 2, a recording layer 3, an upper heat-resistantprotection layer 4, and a reflective heat radiation layer 5 aresequentially stacked in this order. Although the lower protection layer2 is not essential, it should preferably be provided when the substrate1 is made of polycarbonate resin or similar material which is not highlyheat-resistant.

[0033] The protection layers 2 and 4 may be formed by any one ofconventional gaseous phase growth processes including vacuum plating,sputtering, plasma CVD (Chemical Vapor Deposition), optical CVD, ionplating, and electron beam plating. The protections layers 2 and 4should preferably be 100 Å to 5,000 Å thick each, more preferably 200 Åto 3,000 Å each. Protection layers thinner than 100 Å cannot play theexpected role while protection layers thicker than 5,000 Å reduce thesensitivity and are apt to bring about separation at interfaces. Theprotection layers 2 and 4 may each be provided with a laminatestructure, if desired.

[0034] The recording layer contains at least Ag, In, Sb and Te and mayadditionally contain oxygen and/or nitrogen. While the recording layeris, in many cases, amorphous in the event of film formation, it isinitialized by light or heat after the fabrication of the medium. As aresult, a crystal phase AgSbTe₂ and an amorphous phase In-Sb existtogether; AgSbTe₂ has a crystal diameter of less than 100 Å inclusiveand has a stoichiometric composition or a crystallite state closethereto. The crystal phase AgSbTe₂ and amorphous In-Sb are sometimesentangled in a complicated structure.

[0035] The recording layer has the above structure not only when it isfresh after initialization, but also when data stored therein areerased.

[0036] The mixed phase state described above is achievable if AgInTe₂and Sb are implemented as raw materials. In the event of film formation,the recording layer presumably has an AgInTe₂ and Sb amorphous phase dueto the chemical structures of the raw materials. This stems from thefact that when such a recording layer is heated at a temperature aroundthe crystallization transition point (190° to 220° C.), AgInTe2 and Sbcrystal phases are produced. When the recording layer is initialized bya laser beam of adequate power, heat or the like, the crystalliteAgSbTe₂ and amorphous phase In-Sb existing in a uniform mixture isobtained.

[0037] The recording layer may be formed by any one of conventionalgaseous phase growth schemes including vacuum plating, sputtering,plasma CVD, optical CVD, ion plating, and electron beam plating. Apartfrom the gaseous phase growth schemes, use may be made of a sol-gelprocess or similar process using liquid. The recording layer shouldpreferably be 100 Å to 10,000 Å thick, more preferably 150 Å to 3,000 Åthick. Recording layers thinner than 100 Å are extremely low in lightabsorption and cannot play the expected role. Recording layers thickerthan 10,000 Å are difficult to cause uniform phase variation to occur ata high speed.

[0038] For the reflective heat radiation layer, use may be made of Al,Au or similar metal or an alloy thereof. Although the heat radiationlayer is not essential, it is preferable because it radiates excess heatand thereby reduces the thermal load on the disk. The heat radiationlayer may also be formed by any one of conventional gaseous phase growthprocesses including vacuum plating, sputtering, plasma CVD, optical CVD,ion plating, and electron beam plating.

[0039] While the substrate is usually formed of glass, ceramics orresin, resin is advantageous over the others in respect of moldabilityand cost. Typical resins are polycarbonate resin, acryl resin, epoxyresin, polystyrene resin, acrylonitrile-styrene copolymer resin,polypropylene resin, silicone-contained resin, fluorine-contained resin,ABS resin, and urethane resin. Among them, polycarbonate resin and acrylresin are preferable in respect of treatment and optical characteristic.Further, the substrate may be implemented as a disk, card or sheet, asdesired.

[0040] To record, reproduce and erase data from the medium of thepresent invention, there may be used any desired kind of electromagneticwave, e.g., a laser beam, electron beam, X rays, ultraviolet rays,visible rays, infrared rays, or microwave. However, a miniature andcompact semiconductor laser is optimal because it can be easily mountedto a drive.

[0041] The present invention will be described in relation to specificexamples although the former is not limited by the latter.

Examples 1-20 and Comparative Examples 1-20

[0042] A substrate was implemented as a disk and formed ofpolycarbonate. An about 2,000 Å thick lower heat-resistant protectionlayer, an about 200 Å thick recording layer, an about 200 Å upperheat-resistant protection layer and an about 1,000 Å thick reflectiveheat radiation layer were sequentially formed on the substrate by rfmagnetron sputtering. The recording layer was formed of Ag-In-Sb-Te₂while the heat radiation layer was formed of an Al alloy. Thecomposition ratio of the recording layer was changed in conformity tothe thermal conductivity of the protection layers. The protection layerseach consisted of a basic material and a compound having a thermalconductivity of higher than 10 W/m.deg inclusive. Table 1 shown belowlists various combinations of the basic material and compound. TABLE 1Protection Layer Record Erasure Basic Com- Power Ratio RepetitionMaterial pound x (W) (dB) Stability Comp.Ex.1 SiO₂ Al₂O₃ 0.04 18 42 XEx.1 SiO₂ Al₂O₃ 0.37 14 35 ◯ Ex.2 SiO₂ Al₂O₃ 0.7 13 30 ◯ Comp.Ex.2 SiO₂Al₂O₃ 0.97 22 22 Δ Comp.Ex.3 SiO₂ ALN 0.005 19 40 X Ex.3 SiO₂ ALN 0.0515 35 ◯ Ex.4 SiO₂ ALN 0.3 13 32 ◯ Comp.Ex.4 SiO₂ ALN 0.6 25 24 ΔComp.Ex.5 SiO₂ GaN 0.005 19 39 Δ Ex.5 SiO₂ GaN 0.03 14 32 ◯ Ex.6 SiO₂GaN 0.15 13 29 ◯ Comp.Ex.6 SiO₂ GaN 0.35 25 20 X Comp.Ex.7 SiO₂ MgO 0.0218 45 X Ex.8 SiO₂ MgO 0.36 12 36 ◯ Comp.Ex.8 SiO₂ MgO 0.5 13 32 ◯ Ex.7SiO₂ MgO 0.7 20 24 Δ Comp.Ex.9 SiO₂ Si₃N₄ 0.08 18 41 Δ Ex.9 SiO₂ Si₃N₄0.22 12 34 ◯ Ex.10 SiO₂ Si₃N₄ 0.47 13 29 ◯ Comp.Ex.10 SiO₂ Si₃N₄ 0.9 2120 Δ Comp.Ex.11 SiO₂ SiC 0.03 21 40 X Ex.11 SiO₂ SiC 0.2 15 35 ◯ Ex.12SiO₂ SiC 0.43 14 33 ◯ Comp.Ex.12 SiO₂ SiC 0.55 23 22 Δ Comp.Ex.13 SiO₂TiC 0.08 18 37 X Ex.13 SiO₂ TiC 0.16 13 30 ◯ Ex.14 SiO₂ TiC 0.79 14 28 ◯Comp.Ex.14 SiO₂ TiC 0.9 23 21 Δ Comp.Ex.15 SiO₂ TiO₂ 0.08 17 35 X Ex.15SiO₂ TiO₂ 0.35 14 30 ◯ Ex.16 SiO₂ TiO₂ 0.81 12 28 ◯ Comp.Ex.16 SiO₂ TiO₂1 18 20 X Comp.Ex.17 SiO₂ Y₂O₃ 0.08 18 45 X Ex.17 SiO₂ Y₂O₃ 0.34 15 40 ◯Ex.18 SiO₂ Y₂O₃ 0.73 15 38 ◯ Comp.Ex.18 SiO₂ Y₂O₃ 0.95 21 30 ΔComp.Ex.19 SiO₂ ZnO 0.02 16 38 X Ex.19 SiO₂ ZnO 0.18 12 33 ◯ Ex.20 SiO₂ZnO 0.39 14 29 ◯ Comp.Ex.20 SiO₂ ZnO 0.6 18 23 Δ

[0043] In Table 1, the composition ratio x is the molar ratio occupiedby the compound whose thermal conductivity is higher than 10 W/m.deginclusive. In accordance with the present invention, all the recordinglayers had the amorphous layer in the event of production of the disks.The recording layers were initialized (stabilized) by sufficientcrystallization caused by a semiconductor laser beam having a wavelengthof 780 nm. An EFM modulated random pattern was repeatedly overwritten ineach recording layer at a linear velocity of 1.2 m/s to 5.6 m/s in orderto evaluate the characteristic of the medium. The above Table 1 alsolists recording power, erasure ratio and repetition characteristicdetermined with each medium. The erasure ratios were determined in termsof the erasure ratios of a 3T signal of EFM when the 3T signal wasoverwritten by an 11T signal. The repetition characteristic was definedby the number of times at which the block error rate exceeds 220 cps. InTable 1, circles, triangles and crosses are respectively representativeof 100 times and above, ten times and above, and less than 10 times.This is also true with Tables 2 and 3 which will be shown later. It willbe seen that a desirable disk characteristic is achievable with thecomposition range of the present invention. In addition, high recordingdensity is achievable with the present invention, as listed in Table 1.

Comparative Examples 21-23

[0044] Disks each having a protection layer implemented by a ZrO₂ andSiO₂ mixture were produced and evaluated in the same manner as in theprevious examples and comparative examples. Table 2 shown below liststhe results of evaluation. TABLE 2 Protection Layer Basic Com- RecordErasure Repetition Material pound x Power (w) Ratio (dB) StabilityComp.Ex.21 SiO₂ ZrO₂ 0.2 18 35 X Comp.Ex.22 SiO₂ ZrO₂ 0.6 19 37 XComp.Ex.22 SiO₂ ZrO₂ 0.8 20 38 X

[0045] As shown in Table 2, because the thermal conductivity of ZrO₂ isas low as 1.95 W/m.deg in the bulk state, the stable repetitioncharacteristic is not achievable.

Comparative Examples 24-26

[0046] Table 3 shown below lists some different combinations of theupper and lower protection layers and the resulting diskcharacteristics. TABLE 3 Thermal Conductivity Lower of Lower UpperProtection Protection Protection Repetition Layer Layer Layer StabilityComp.Ex.24 SiO₂ 1.2 Al₂O₃ ◯ Comp.Ex.25 Al₂O₃ 25 Al₂O₃ Δ Comp.Ex.26 ZnO50 Al₂O₃ X

[0047] As shown, when the lower protection layer has a higher thermalconductivity than the upper protection layer, the stable repetitioncharacteristic is not achievable.

[0048] In summary, it will be seen that the present invention provides aphase variation type optical data recording medium having a high erasureratio and allowing data to be repeatedly recorded and erased a number oftimes by small power even at the time of high-speed recording anderasure.

[0049] Various modifications will become possible for those skilled inthe art after receiving the teachings of the present disclosure withoutdeparting from the scope thereof.

What is claimed is:
 1. In a material for a heat-resistant protectionlayer and constituting one of a plurality of components of a phasevariation type recording medium, at least one compound having a thermalconductivity of higher than 10 W/m.deg inclusive in a bulk state iscontained.
 2. A material as claimed in claim 1 , wherein said at leastone compound is selected from a group consisting of zinc oxide, aluminumoxide, titanium oxide, magnesium oxide, yttrium oxide, gallium nitride,silicon nitride, aluminum nitride, and silicon carbide.
 3. A material asclaimed in claim 1 , wherein said at least one compound comprises acombination of zinc oxide, aluminum oxide, titanium oxide, magnesiumoxide, yttrium oxide, gallium nitride, silicon nitride, aluminum nitrideand/or silicon carbide, and silicon oxide.
 4. In an optical datarecording medium comprising a substrate and a heat-resistant protectionlayer, a recording layer and a reflective heat radiation layersequentially stacked on said substrate, said recording layer mainlyconsists of Ag, In, Sb and Te, and said heat-resistant protection layercontains at least one compound having a thermal conductivity of higherthan 10 W/m.deg inclusive in a bulk state.
 5. A material as claimed inclaim 4 , wherein said at least one compound is selected from a groupconsisting of zinc oxide, aluminum oxide, titanium oxide, magnesiumoxide, yttrium oxide, gallium nitride, silicon nitride, aluminumnitride, and silicon carbide.
 6. A material as claimed in claim 4 ,wherein said at least one compound comprises a combination of zincoxide, aluminum oxide, titanium oxide, magnesium oxide, yttrium oxide,gallium nitride, silicon nitride, aluminum nitride and/or siliconcarbide, and silicon oxide.